# this is the code
import os
import subprocess
import ipdb

root_dir = '/home/pratz/Desktop/music/'
target_dir = '/home/pratz/Desktop/test_music/'

for root, subfolder, files in os.walk(root_dir) :

    ipdb.set_trace()

    for fl in files :
        if ".mp3" in fl :
            target = target_dir + fl.replace(' ','_')
            compress_ffmpeg = ["ffmpeg","-i",fl,"-ab","128k", target]
            subprocess.call(compress_ffmpeg,shell=True)


# And this is the dir structure ( ipython ) 

In [6]: test_walk = os.walk("/home/pratz/Desktop/music/")

In [7]: [ (x,y,z) for x,y,z in  test_walk]
Out[7]: 
[('/home/pratz/Desktop/music/', ['akon'], []),
 ('/home/pratz/Desktop/music/akon', ['Freedom [2008]'], []),
 ('/home/pratz/Desktop/music/akon/Freedom [2008]',
  [],
  ['14-Akon-Clap Again (BONUS).mp3',
   '12-Akon-Over The Edge.mp3',
   "05-Akon-We Don't Care.mp3",
   '01-Akon-Right Now (Na Na Na).mp3',
   '03-Akon-Keep You Much Longer.mp3',
   '04-Akon-Troublemaker Ft. Sweet Rush.mp3',
   '08-Akon-Against The Grain Ft. Ray Lavender.mp3',
   '11-Akon-Birthmark.mp3',
   '10-Akon-Sunny Day Ft. Wyclef.mp3',
   '13-Akon-Freedom.mp3',
   '07-Akon-Holla Holla Ft. T-Pain.mp3',
   "06-Akon-I'm So Paid Ft. Lil' Wayne And Young Jeezy.mp3",
   '09-Akon-Be With You.mp3',
   '00-Akon-Freedom-2008-[NoFS]-COVER.jpg',
   "02-Akon-Beautiful Ft. Colby O'Donis And Kardinal Offishall.mp3"])]

# when i run the program i am getting the error
FFmpeg version SVN-r0.5.1-4:0.5.1-1ubuntu1.3, Copyright (c) 2000-2009 Fabrice Bellard, et al.
  configuration: --extra-version=4:0.5.1-1ubuntu1.3 --prefix=/usr --enable-avfilter --enable-avfilter-lavf --enable-vdpau --enable-bzlib --enable-libgsm --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-pthreads --enable-zlib --disable-stripping --disable-vhook --enable-runtime-cpudetect --enable-gpl --enable-postproc --enable-swscale --enable-x11grab --enable-libdc1394 --enable-shared --disable-static
  libavutil     49.15. 0 / 49.15. 0
  libavcodec    52.20. 1 / 52.20. 1
  libavformat   52.31. 0 / 52.31. 0
  libavdevice   52. 1. 0 / 52. 1. 0
  libavfilter    0. 4. 0 /  0. 4. 0
  libswscale     0. 7. 1 /  0. 7. 1
  libpostproc   51. 2. 0 / 51. 2. 0
  built on Dec 21 2011 18:37:21, gcc: 4.4.3
At least one output file must be specified